The present invention relates to a roll caster shell for use in a continuous sheet casting machine.
Continuous casting of the twin-roll type is a process in which a molten metal is poured directly into the gap between a pair of rolls which rotate in opposite directions from each other so as to produce a sheet with a thickness of 0.5-10 mm. Since this process can result in savings in manufacturing steps and equipment and has a possibility of creating a new product, the twin-roll type continuous casting process has been widely used in the manufacture of thin aluminum sheets. Because of its advantages, many efforts have recently been made to apply such a process to the manufacture of steel sheets as well.
To elucidate the basic idea, two-roll type continuous casting process will be described in detail with reference to aluminum casting.
FIG. 1 is a schematic illustration of the twin-roll type continuous casting process in which a pair of rolls 10, 10 having a water-cooled double-walled structure is usually employed in order to promote solidification of the molten metal 12 poured into the roll gap through a header 14. Casting nozzle 15, 15 form a guide to the roll gap. Each roll 10 comprises a shell portion 16 which directly contacts the molten metal and a core portion 18 which has a groove 20 formed in its outer surface as a passage for cooling water 22. The roll 10 is assembled by shrink-fitting the shell 16 onto the core 18 or by connecting the shell and core with screws after inserting the core 18 into the shell 16.
During casting, the roll shell is alternately subjected to heating by molten metal and then cooling by cooling water, resulting in formation of heat cracks due to thermal fatigue on the surface of the shell. Thus, the shell must be machined to remove surface cracks when the heat cracks on the surface of the shell become so severe as to damage the surface quality of the cast sheets. This results in additional labor and material costs.
Thus, the material of the roll shell must have an improved resistance to heat cracking. Furthermore, such a material must have excellent thermal conductivity. When a material of low conductivity is used to manufacture a roll shell, productivity is lower because the casting speed must be lowered since solidification of a molten metal within a roll gap is slower.
As is well known, the thermal conductivity of steel degrades as the content of alloying elements increases, so a roll shell for use in continuous casting must be made of a material which has a relatively small amount of alloying elements and exhibits improved resistance to heat cracking during casting.
Conventional materials include:
(i) A steel material having an alloy composition which consists essentially of 0.53-0.58% of C, 0.20-0.30% of Si, 0.45-0.65% of Mn, 0.02% or less of P, 0.02% or less of S, 0.40-0.50% of Ni, 1.0-1.2% of Cr, 0.45-0.55% of Mo, 0.10-0.15% of V, and a balance of Fe and incidental impurities. This steel will be referred to as Conventional Steel I. PA1 (ii) A steel material disclosed in U.S. Pat. No. 4,409,027, which consists essentially of 0.53-0.58% of C, 0.10-0.20% of Si, 0.40-0.70% of Mn, 0.02% or less of P, 0.02% or less of S, 0.45-0.55% of Ni, 1.90-2.30% of Cr, 0.9-1.1% of Mo, 0.30-0.35% of V, and a balance of Fe and incidental impurities. This steel will be referred to as Conventional Steel II. PA1 (iii) An alloy steel disclosed in a French reference "Steels for aluminum continuous caster shells", Bull Cercle Etud Metaux, Vol. 15, No. 10 '85, which consists of 0.32% of C, 0.5% of Mn, 0.3% of Ni, 3% of Cr, 1% of Mo, 0.2% of V, and a balance of Fe and incidental impurities. This alloy steel will be referred to as Conventional Steel III. PA1 C: 0.35-0.55%, Si: 0.10-0.50%, Mn: 0.20-0.70%, PA1 P: 0.03% or less, S: 0.02% or less, Ni: 0.60% or less, PA1 Cr: 0.80-1.50%, Mo: 0.80-1.50%, V : 0.30-0.60%, PA1 Co: 0.30-1.00%, PA1 Fe and incidental impurities: balance
Due to an increasing casting speed and a decrease of thickness of aluminum sheet, both of which have recently been in demand, a roll shell with more improved resistance to heat cracking and higher thermal conductivity than these conventional materials is greatly needed.